code: def sampling_group_systematic(n):
      """
      Return a subset where groups are selected at a regular interval.
  
      Parameters
      ----------
      n : int
          Number of groups to return.
  
      Returns
      -------
      `FeatureSpec`
          Group systematic sample specification.
  
      Examples
      --------
      .. jupyter-execute::
          :linenos:
          :emphasize-lines: 11
  
          import numpy as np
          from lets_plot import *
          LetsPlot.setup_html()
          waves_count = 100
          peak_amplitude = np.linspace(1, 2, waves_count)
          wave_x = np.linspace(-np.pi, np.pi, 30)
          x = np.tile(wave_x, waves_count)
          y = np.array([a * np.sin(wave_x) for a in peak_amplitude]).flatten()
          a = np.repeat(peak_amplitude, wave_x.size)
          ggplot({'x': x, 'y': y, 'a': a}, aes('x', 'y')) + \\
              geom_line(aes(group='a', color='a'), sampling=sampling_group_systematic(10))
  
      """
  
      return _sampling("group_systematic", n=n)
  
file_path: lets-plot/python-package/lets_plot/plot/sampling.py
imports: from .core import FeatureSpec
label_desc: Use image node_id 5 for calling a global function with example usage: sampling_group_systematic(n) and returns: _sampling